Although the invention is not necessarily limited thereto, it is particularly suitable for use and transmission of subscription TV programs by cable systems. The invention may also be applied wherever sync suppression scrambling and descrambling of television signals is used whether the transmission link is a cable, broadcast, or satellite link.
Sync suppression scrambling and descrambling of television signals is a well known and accepted technique for encoding and decoding subscription television signals where the amplitude of the vertical or horizontal synchronizing (sync) signals, or both, is attenuated so as to prevent a television receiver from locking onto the signal. The picture received by a subscriber receiver not equipped with a descrambler rolls and is so distorted so as to make viewing impossible or uncomfortable. Reference may be had to U.S. Pat. No. 4,466,017 issued to Robert Banker on Aug. 14, 1984 for further information as to sync suppression scrambling and descrambling of television signals. The disclosure of Banker is incorporated herein by reference.
Briefly described, sync suppression scrambling and descrambling of TV signals for subscription TV programs may be carried on by generating sync suppression pulses in overlapping time relationship with the horizontal and/or vertical sync pulses of the TV signal. Timing pulses are generated in selected time relationship with the sync suppression pulses such that each of the timing pulses and the sync suppression pulses have a selected time delay with respect to each other. This time delay may be selected, preferably digitally, from a plurality of different delays or dynamically varied. The horizontal and/or vertical sync pulses in the television signal are thereby suppressed below their normal amplitude. Sync detectors of conventional TV receivers at the subscriber are incapable of locking onto the attenuated sync pulses of the transmitted television signal. The timing pulses are transmitted with the TV signals, preferably by modulating the IF audio carrier thereof, during encoding of the TV signals. Scrambled and encoded TV signals are transmitted from a transmitting station which may be, for example, the headend of a cable TV system, to the receiving stations.
The timing pulses are derived at these receiving stations and the selected delay added to generate delayed timing pulses. Then restoring pulses are generated in proper overlapping time relationship with the received horizontal sync pulses upon the occurrence of the delayed timing pulses. The TV signals are restorable to normal amplitude through the use of the restoring pulses to descramble the TV signals. Therefore, the receiving station to restore the pulses in synchronism with the horizontal sync intervals of the TV signals must be not only authorized to receive the scrambled signals but also be tuned to the particular premium channel which is scrambled. Other premium channels which are scrambled are usually out of phase with a particular timing signal and cannot be descrambled without being tuned. In the past the duplication of the tuner in a converter/descrambler and a cable ready TV was considered inevitable.
After tuning in the particular TV premium channel which is to be descrambled and decoding the timing pulses, the descrambling process is one of restoring the horizontal sync pulses to their original level relative to the rest of the TV signal. This is generally accomplished by a controlled impedance device, either an attenuator or an amplifier, which is placed in series with the signal and at the correct times switched on and off to vary its impedance and thus the amplitude of the synchronization pulses relative to the rest of the signal.
Conventional descramblers used in subscription cable television to descramble the sync suppression signals from a headend usually contain a converter or tuner. In many, the output frequency of the CATV converter is fixed, such as channels 2, 3, or 4, requiring that all channel changing must be done through the CATV converter. In addition, this is a redundant function where the CATV converter has a tuner whose operation closely duplicates that of the tuner in a cable ready television receiver and adds a significant increase to the cost of the CATV converter along with some degradation of the TV signal.
An additional problem in developing descramblers which are compatible with the present CATV systems is the compatibility of signaling methods. Authorization information and data for other operations are transmitted in band and out of band for various systems. An in band signaling system frequently utilizes the sound carrier by modulating it with timing and data pulses. This does not affect the sound, which is frequency modulated on the carrier. For out of band systems, a single FM modulated signal is set aside for authorization messages with no timing data included thereon. It would be highly advantageous for a descrambling apparatus to be compatible with the CATV system in which it is placed without having to replace other converters because of incompatible message protocols.